Project Summary: Hematopoietic stem cell (HSC) transplantation (HST) is a curative treatment for many hematopoietic malignancies. However, a potentially fatal complication of HST is the high risk of infection and bleeding recipients are exposed to during the up to 4 weeks while awaiting regeneration of peripheral blood neutrophils and platelets. I and collaborators published in 2015 in Science the discovery of a small molecule, SW033291, that potently inhibits the prostaglandin degrading enzyme 15-PGDH and markedly speeds recovery from HST. As co-first author, I led studies that showed that treating with SW033291 during murine HST enhances transplanted stem cell homing to the bone marrow niche; markedly accelerates recoveries of neutrophils, platelets, and erythrocytes; and increases the effective dose of a bone marrow graft by 5-fold. I showed these effects are mediated by increased bone marrow PGE2, which acts on EP2 and EP4 PGE2 receptors to induce key hematopoietic cytokines, SCF and CXCL12, in bone marrow stromal cells. In this application I now propose elucidate the full cellular and signaling mediators of SW033291's effect on HST and to examine the interaction of SW033291 with G-CSF, the clinical standard of care for stimulating hematopoietic recovery after human HST. I will first comprehensively identify the molecular mediators of SW033291 effect (using RNA-seq) and identify the bone marrow cells in which these mediators are induced and act (e.g. identifying the novel 15-PGDH positive bone marrow cell that is first targeted by SW033291 and then the downstream cells in which SCF and CXCL12 are markedly induced). I will second interrogate the in vivo function of candidate mediators of SW033291 effect, first by testing the ability of this drug to potentiate HST when candidate SW033291 mediators (e.g. 15-PGDH, CXCL12, SCF, plus two new candidate mediators CXCL4 and CXCL7) are genetically deleted from both donor mouse stem cells and recipient mouse bone marrow stroma. I will further map the differential effects of these mediators on hematopoietic stem cells versus bone marrow stromal cells by examining the ability of SW033291 to potentiate HST when candidate mediators are selectively deleted only in donor HSCs or only in recipient bone marrow stromal cells. Using these models I will further dissect each mediator's role at each of three successive stages of SW033291 effect that are: i) potentiating homing of donor HSCs to the recipient marrow; ii) inducing hematopoietic cytokines in the marrow HSC niche; iii) potentiating stem cell generation of daughter cells. Third, I will examine the interaction of SW033291 with G-CSF, the standard of care used to augment neutrophil recovery in human HSC, following up preliminary data that shows an additive to synergistic effect of SW033291 combined with G-CSF, and characterizing the mechanism of this synergy through identification of target changes in the transcriptome and phosphoproteome of selected bone marrow populations from mice treated with these agents singly or in combination. Through these studies I will stepwise acquire the experience and skills that will launch me as new and fully independent investigator.